1000? What happens if you drop your ADU down to 1? My understanding is that the Analogue to Digital Unit converts the Analogue signal to digital and the that by setting the ADU to 1000 it will take 1000 electrons to convert to 1 ADU.
Unless I'm misunderstanding what you are doing. Afaik with the SX camera I use the gain is set at the factory (don't quote me though). The QHY8 has adjustable gain?